3u + z = 15, (1) u + 2z = 10. (2) Substitute u = 10 - 2z from (2) into (1). You will get a single equation for u: 3*(10-2z) + 2z = 10. Simplify and solve it: 30 - 6z + 2z = 10, 30 - 10 = 4z, 4z = 20, z = = 5. Now find u from (2).